The ruins of the city of Cologne's historical archive, which collapsed on Tuesday 3 March 2009. Photograph: Vladimir Rys/Getty

For the best part of a decade, the heirs of German writer and Nobel prize laureate Heinrich Böll worked on hammering out a deal with the city of Cologne over the transfer of his private papers to the state archives.

Three weeks ago, city officials held a special ceremony to mark the historic handover: for €800,000 (£712,000), the Cologne archives took possession of hundreds of boxes containing items ranging from Böll's school reports to scripts of his radio plays, novels and essays by Germany's most popular post-second world war writer, who died in 1985 at the age of 67.

But his papers and unpublished works may have been lost for ever after the collapse of the archives building this week.

The six-storey building was demolished on Tuesday after its foundations caved in under the strain of a nearby building project.

Excavation experts are still searching the site for two people believed to have been caught in the collapse.

They say it is probable that the papers have been lost under the concrete, steel and glass rubble.

Lost, too, were photographs and 80,000 letters - including 2,400 written to his wife Annemarie - all of which had been stored for years in a damp cellar of the Bölls' home.

Experts fear that even if the documents were not entirely crushed in the collapse, ground water and soil which has seeped into the hole left by the destroyed foundations will have ruined them. Restoration experts said the longer it took rescue workers to remove the rubble, the higher the danger that mould would attack the manuscripts.

It is not believed that any of the papers had been copied in digital form or on to microfilm.

The loss to academics and literary historians has been compared to the destruction caused by wartime bombing raids. Johannes Fried, a medievalist from Frankfurt, said the collapse of the archives - which also housed one of the most important collections of medieval documents - was a "catastrophe for all European historians".

Helge Malchow, the head of Kiepenheuer und Witsch, the publisher of Böll's works -including The Clown, Group Portrait with Lady and The Lost Honour of Katharina Blum - confirmed that all the writer's unpublished works were held in the archives. Their total loss would be "unquantifiable", he said. "If these materials, manuscripts and other original documents really are lost, it will create a hole that will probably never be able to be filled. We and the Böll researchers are feeling numb."

Böll's 60-year old son, who is travelling in China, was said to be horrified by the catastrophe.

One point of solace for Böll enthusiasts is the recent completion of a 27-volume collection of the writer's complete works which will be published next year. The collection was dependent for its sources on works held in the Cologne archives.

"If the unpublished works have been lost, this work will assume a most incalculable value," said Malchow.

The Böll documents are just a small part of the losses to the archives which contained almost 30km of files, including articles written by Karl Marx, letters by Georg Hegel, writings by composer Jacques Offenbach and edicts issued by Napoleon Bonaparte, as well as the minutes of city council meetings going back to 1376, which offer a fascinating portrait of medieval Cologne.